Medical devices having MRI compatible metal alloys

ABSTRACT

An MRI compatible medical device includes a non-magnetic metal alloy portion including a first and at least a second metal. A surface of the metal alloy portion includes an integral MRI heating resistant surface structure having a thickness≧3 nanometers. The MRI heating resistant surface structure includes one or more of (i) a matrix phase including the first and second metal having a plurality of nanometer or micron scale particles, precipitates and/or inclusions constituting a volume fraction≧3%, wherein the particles, precipitates or inclusions differ in chemical composition and physical characteristics of the matrix phase and are discontinuously distributed therein; (ii) a level of crystallinity at least 5% less as compared to a level of crystallinity in the bulk of the metal alloy portion; (iii) one or more metal atoms different from the first and second metal having a concentration profile evidencing diffusion into the metal alloy portion.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Provisional Application Ser. No.61/111,091 entitled “METALLIC ALLOYS AND DEVICES WITH IMPROVED MRICOMPATIBILITY AND METHODS FOR FABRICATING THE SAME”, filed Dec. 2, 2008,which is herein incorporated by reference in its entirety.

FIELD

Disclosed embodiments relate to medical devices including non-magneticmetal alloys, including methods for fabricating such alloys, wherein thealloys provide enhanced MRI compatibility.

BACKGROUND

Magnetic Resonance Imaging (MRI) is a commonly used diagnostic tool toimage soft tissues in the human body. MRI systems use strong magneticfields. The strong magnetic fields can attract ferromagnetic objectswith significant force and torque while the pulsed gradient and radiofrequency (RF) fields may induce currents that can produce significantheating in metallic objects. For example, Sommer et al. (Sommer T,Vahlhaus C, Lauck G, von Smekal A, Reinke M, Hofer U, Block W, Traber F,Schneider C, Gieseke J, Jung W, Schild H: MR imaging and cardiacpacemakers: in-vitro evaluation and in-vivo studies in 51 patients at0.5 T Radiology 2000, 215(3):869-79.4) demonstrated the potential forinduced heating as much as 23.5° C. at specific absorption rate (SAR)levels of only 1.3 W/kg in a 0.5 Tesla MRI unit.

A desirable patient for an MRI procedure (e.g. MRI scan) is one who doesnot have any metallic medical devices implanted in his or her body thatwould thus avoid interaction with the magnetic and RF fields associatedwith the MRI scan. However, a significant percentage of potentialpatients for MRI have implanted medical devices such as pacemaker leads,stents, clips, plates, and joint prostheses. Even if the metal in theimplanted device is non-ferromagnetic, the RF field associated with theMRI procedure can lead to harmful heating effects as a result of the RFinduced currents in the metal. Hence there is a need for metal alloysfor medical devices that are less susceptible to such harmful heatingeffects. Conventional solutions to the MRI interaction problem havegenerally relied on coatings or resonator technologies external to theimplanted device, such as copper windings.

MRI can also be used for imaging of implanted medical devices during amedical procedure. Medical devices are generally defined as productsused for medical purposes in patients, in diagnosis, therapy or surgery.If applied to the body, the effect of the medical device is primarilyphysical, in contrast to pharmaceutical drugs, which exert a biochemicaleffect.

Examples of imaging of implanted medical devices during a medicalprocedure include real-time imaging while placing a stent, using acatheter or other medical device in the human body. It may also includepost-implant imaging of the implanted stent or other medical deviceduring the life of the patient. In such cases the medical implant maynot be visible clearly in MRI resulting in imaging artifacts such asareas of poor or no contrast, or in some cases even not providingvisibility through the device. An example is struts of a stent which donot allow for any image of the tissue or blood vessel between thestruts, but the entire stent images without any difference between thestrutted and the non-strutted region. Hence there is also a need formetal alloys for medical devices that are less susceptible to suchimaging artifacts and poor visibility.

The availability of medical devices having enhanced MRI compatibilitydevices could enable their extension and incorporation into a suite ofmedical instruments (e.g., guide wires, catheters, needles, etc.) thatwould facilitate MRI interventional procedures without the need toexamine the patient for possible different types of contraindicationsthat would prevent a person from being examined with an MRI scanner.Such procedures could also replace more traditional fluoroscopicprocedures, thereby minimizing the patient's and the physician'sexposure to harmful radiation.

SUMMARY

This Summary is provided to comply with 37 C.F.R. §1.73, presenting asummary of this disclosure to briefly indicate the nature and substanceof the subject matter disclosed herein. It is submitted with theunderstanding that it will not be used to interpret or limit the scopeor meaning of the claims.

Embodiments of the invention describe MRI compatible medical devicesthat have at least one non-magnetic metal alloy portion. The metal alloyportion comprises a first and at least a second metal different from thefirst metal. A surface of the metal alloy portion includes an integralMRI heating resistant surface structure that has a thickness≧3nanometers. MRI heating resistant surface structures can also beconfigured to increase MRI visibility, generally by adding suitableadditional metals, such as Au, Pt, Pd or Ag.

Significantly, the skin depth of the interaction between the RF waveduring MRI has been found by the Inventors to be altered by the MRIheating resistant surface structure of the metallic alloy or device soas to reduce the energy absorbed during MRI that is available to inducea current, thus reducing heating. Moreover, it has also been found bythe Inventors that the MRI heating resistant surface structure of themetallic alloy or device has an altered impedance that results inreduced heating from the current induced by the RF field.

The MRI heating resistant surface structure comprises at least one of:

(i) a matrix phase comprising the first and second metal having aplurality of nanometer scale to micron scale particles, precipitatesand/or inclusions therein constituting a volume fraction≧3%, wherein theparticles, precipitates or inclusions differ in chemical composition andphysical characteristics from the matrix phase and are discontinuouslydistributed therein;

(ii) a level of crystallinity that is at least 5% less as compared to alevel of crystallinity in a bulk of the metal alloy portion; and

(iii) one or more metal atoms different from the first and second metalhaving a concentration profile evidencing diffusion into the metal alloyportion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a depiction of an exemplary partially cut open medical devicethat includes a battery, wherein a metal alloy portion includes an MRIheating resistant surface structure that provides leads that are coupledto the battery for carrying current from the battery, while FIG. 1B is across sectional depiction of a wire in a lead shown in FIG. 1A showing abulk that is compositionally and/or structurally different from the MRIheating resistant surface structure, according to a disclosedembodiment.

FIG. 2A is a depiction of an exemplary non-current carrying medicaldevice, wherein a metal alloy portion includes an MRI heating resistantsurface structure, while FIG. 2B shows bulk that is compositionallyand/or structurally different from the MRI heating resistant surfacestructure, according to a disclosed embodiment.

FIG. 3 shows a depiction of a metal alloy material having ananostructure or microstructure that includes both continuous anddiscontinuous lamellar structures formed by a spinodal decompositionprocess, according to an embodiment of the invention.

FIG. 4 shows a depiction of a metal alloy material that includes anouter surface layer having a high concentration of inclusions and arecast layer below the outer surface layer having a different level ofcrystallinity, according to another embodiment of the invention.

FIG. 5 is a depiction showing the microstructure of an exemplary outersurface layer according to another embodiment of the invention. Theouter surface layer can be seen to include a plurality of precipitates,inclusions and particles.

FIG. 6 is a depiction of a concentration profile of diffused atoms for ametal alloy material according to another embodiment of the invention.

DETAILED DESCRIPTION

Disclosed embodiments are described with reference to the attachedfigures, wherein like reference numerals are used throughout the figuresto designate similar or equivalent elements. The figures are not drawnto scale and they are provided merely to illustrate the disclosedembodiments. Several aspects are described below with reference toexample applications for illustration. It should be understood thatnumerous specific details, relationships, and methods are set forth toprovide a full understanding of the disclosed embodiments. One havingordinary skill in the relevant art, however, will readily recognize thatthe disclosed embodiments can be practiced without one or more of thespecific details or with other methods. In other instances, well-knownstructures or operations are not shown in detail to avoid obscuringcertain details. The disclosed embodiments are not limited by theillustrated ordering of acts or events, as some acts may occur indifferent orders and/or concurrently with other acts or events.Furthermore, not all illustrated acts or events are required toimplement a methodology in accordance with the disclosed embodiments.

Disclosed embodiments comprise medical devices having at least onenon-magnetic metal alloy portion, wherein the metal alloy portioncomprises a first and at least a second metal different from the firstmetal. A surface of the metal alloy portion includes an integral MRIheating resistant surface structure that has thickness≧3 nanometers toprovide improved MRI compatibility for the medical device. The heatgenerating reducing layer is an integral layer that is integral with themetal alloy or device since it is provided by processing performed tothe metal alloy or device described below, as opposed to non-integrallayer(s) that are deposited onto the metal alloy or device in the formof a coating, or are adhesively joined or fastened, or wound around.

Exemplary medical devices or portions thereof that can benefit fromdisclosed embodiments include, but are not limited to, those which arecoupled to a power supply such as a battery and are current carryingsuch as cardiac leads (e.g., for pacemaker devices), neuro-stimulatorleads (for neuro-stimulator devices), and non-current carrying such asclips, staples, markers, stents, catheters, guide wires, screws, bolts,orthopedic implants, cochlear implants, and valves.

For purposes of describing disclosed embodiments, “improved MRIcompatibility” is defined as follows:

-   1. Reduced heating due to the gradient and RF fields associated with    an MRI environment (when compared to alloys or devices known in the    art and permissible under existing government, e.g. the U.S. FDA    guidelines), as measured by ASTM F2182-02a for Measurement of    Measurement of Radio Frequency Induced Heating Near Passive Implants    During MRI, 2006); and-   2. Reduced imaging artifacts (when compared to alloys or devices    known in the art and permissible under existing government, e.g. the    U.S. FDA guidelines), as measured by ASTM F2119-01 for Evaluation of    MR Image Artifacts from Passive Implants, 2006). Improved MRI    compatibility also generally includes:-   3. Reduced displacement due to a magnetic field associated with an    MRI environment when compared to alloys or devices known in the art    and permissible under existing government, e.g. the U.S. FDA    guidelines), as measured by ASTM F2052-06 for Measurement of    Magnetically Induced Displacement Force on Medical Devices in the MR    Environment and ASTM F2213-06 for Measurement of Magnetically    Induced Torque on Medical Devices in the MR Environment, 2006.

Methods are disclosed herein for fabricating non-magnetic metallicalloys with improved MR compatibility that can be used for medicalimplants and devices, or to improve the MR compatibility of metallicalloys that have already been formed into medical implants and devices.As used herein “non-magnetic” refers to acceptable behavior in amagnetic field following ASTM standard documents F2052-06 forMeasurement of Magnetically Induced Displacement Force on MedicalDevices in the MR Environment (2006), and F2213-06 for Measurement ofMagnetically Induced Torque on Medical Devices in the MR Environment(2006). If the magnetically induced deflection force is less than theforce on the device or metal alloy due to gravity (its weight), it isconsidered acceptable behavior or “non-magnetic” herein since the riskimposed by the application of the magnetically induced force is nogreater than the risk imposed by normal daily activity in the Earth'sgravitational field.

The MRI heating resistant surface structure is provided at the surfaceof the metal alloy or device, and is optionally in its bulk as well. TheMRI heating resistant surface structure comprises at least one of:

(i) a matrix phase comprising said first and said second metal having aplurality of nanometer scale to micron scale particles, precipitatesand/or inclusions therein constituting a volume fraction≧3%, whereinsaid particles, precipitates or inclusions differ in chemicalcomposition and physical characteristics from said matrix phase and arediscontinuously distributed therein;

(ii) a level of crystallinity that is at least 5% less as compared to alevel of crystallinity in a bulk of said metal alloy portion; and

(iii) one or more metal atoms different from the first and second metalhaving a concentration profile evidencing diffusion into said metalalloy portion (see FIG. 6 described below).

As described above, MRI heating resistant surface structure can comprisein any order one or more of (i), (ii) and (iii), and a continuous ordiscontinuous lamellar nanostructure or microstructure structure, whichcan in one embodiment completely extend into the bulk of the metal alloyor device. For example, two or more of (i), (ii) and (iii) can also becombined, as well as with a continuous or discontinuous lamellarnanostructure or microstructure. For example, one or more atoms can bediffused into a matrix phase that already has a plurality of nanometerscale to micron scale particles, precipitates and/or inclusions.

The natural tendency of the atoms is to arrange themselves in a lattice.However, if the alloys are heated and then cooled very quickly in such amanner that the atoms do not have sufficient time to arrange themselvesin a crystal lattice, then they may be considered partly or fullyamorphous. As used herein, “a different level of crystallinity” refersto a change in average % crystallinity of at least 5%, wherein 100%represents a perfect crystal and 0% represents a fully amorphous state.As known in the art, a phase is a uniform portion of an alloy that hasthe same chemical and physical characteristics.

The distribution and size of precipitates, inclusions or particlesrefers to a second phase in the matrix of the first phase. A typicalconcentration range for the precipitates, inclusions or particles is toprovide a volume fraction≧3%. The second phase can range from nanometerscaled to micron scaled particles, precipitates or inclusions. Thesediffer in chemical composition and physical characteristics from thematrix phase and may be a result of a chemical reaction between thealloy and the gases in contact with the surface or inherent products ofthe heat treatment from within the alloy.

The MRI heating resistant surface structure can include a continuous ordiscontinuous lamellar nanostructure or microstructure at the surface orthroughout the bulk of the material (see FIG. 3). The metallic elementsthat constitute the alloys or have been introduced into the alloy aregenerally selected so as to form one or more phases when heated.

When subsequently cooled, one or more of the phase(s) can undergodecomposition into a continuous or discontinuous lamellar nanostructureor microstructure. The decomposition is generally a spinodaldecomposition, eutectic or eutectoid reaction that can occur in a widerange of alloy systems. As known in the metallurgical arts, spinodaldecomposition, eutectic or eutectoid reactions are methods by which amixture of two or more materials can separate (i.e. segregate) intodistinct regions with different material concentrations and formlamellar nanostructures or microstructures. The lamellar nanostructureor microstructure can be produced either on the surface of the alloy ordevice or throughout its bulk.

The elements comprising alloys according to disclosed embodiments areselected so as to produce non-magnetic alloys as defined above. In somecases, one or more ferromagnetic metals may be included in the alloy,provided the resulting alloy is non-magnetic. For example, as known inthe art, Ni is ferromagnetic, but combines with Ti to form NiTi which isa non-magnetic alloy. Elemental additions may also be used that may beferromagnetic provided in a concentration limited by the magneticallyinduced deflection force as a result of their addition remaining lessthan the force on the device or alloy due to gravity (its weight). Theseelements can be included to improve MR visibility and contrast and insome cases even allow for imaging of the tissues in regions betweenparts of the medical device or implant. For example elements such as Agand Pd are known for this purpose in stents (See Lukas C. van Dijk,Jacqueline van Holten, Bastiaan P. van Dijk, Niels A. A. Matheijssen andPeter M. T. Pattynama, A Precious Metal Alloy for Construction of MRImaging—compatible Balloon expandable Vascular Stents, Radiology 2001;219:284-287).

FIG. 1A is a depiction of a partially cut open exemplary medical device100 that includes a battery 105, wherein a metal alloy portion providesleads 110 that include an MRI heating resistant surface structure thatare coupled by connector pins 114 to the battery 105 for carryingcurrent from the battery to one or more electrodes (not shown),according to a disclosed embodiment. Electronic circuitry 120 is alsoshown. FIG. 1B is a cross sectional depiction of one or more wires thatform lead 110 shown in FIG. 1A evidencing a bulk portion 111 that iscompositionally and/or structurally different from the MRI heatingresistant surface structure 112, according to a disclosed embodiment.The leads are generally less than a few mm in diameter with wires in theleads generally being around tens to hundreds of microns in diameter.

Medical device 100 can comprise an implanted current carrying medicaldevice such as a cardiac device (e.g., pacemaker) or a neuro-stimulatordevice. The MRI heating resistant surface structure 112 provides anincrease of at least 10% in the magnitude of its impedance in thefrequency range of the RF field (42.56 MHz per Tesla) as compared to thebulk 111 of the metal alloy portion.

FIG. 2A is a depiction of an exemplary non-current carrying medicaldevice shown as a biopsy needle device 200 for collecting a tissuespecimen, wherein the needle 205 comprises a metal alloy portion thatincludes an MRI heating resistant surface structure, generally along itsentire length. Biopsy needle device 200 also includes a handle body 210that includes handles 211 and 212, and a plunger mechanism 215 that isoperable to create a suction to draw a tissue specimen into the needle205.

FIG. 2B shows a cross sectional depiction of needle 205. Needle 205includes hollow center region 216, a bulk portion 217 and a MRI heatingresistant surface structure 218, according to a disclosed embodiment. Inone embodiment, MRI heating resistant surface structure 218 includes alamellar nanostructure or microstructure, and wherein the lamellarnanostructure or microstructure extends into both the MRI heatingresistant surface structure 218 and the bulk portion 217. The MRIheating resistant surface structure 218 can include one or more metalatoms different from the first and second metal, such as Pt, Pd, Ag orAu, in a concentration of at least 0.1 atomic % (of the metal alloy) forincreasing the MRI visibility of the needle 205. The one or more metalatoms different from the first and second metal, such as Pd or Ag, aregenerally not included in bulk portion 217.

The metal alloy can be processed and thermo mechanically treatedgenerally using metallurgical techniques. Processing may also includeforming (e.g., extruding) or casting the alloy material into a shapesuitable for a device or implant, such as into wire or lead form.

Following forming into a suitable shape, a source of heat can then beapplied to the surface of the metal alloy either in vacuum or in acontrolled gaseous and/or temperature environment. The heating isgenerally performed at a temperature between that where diffusionprocesses are active in the time scale required for heating to wherelocalized melting and/or chemical reaction at or below the surfaceoccurs. Typical temperatures range from a third of the meltingtemperature of the alloy to the melting temperature of the alloy.Examples of such heat sources include a laser beam or wire typicallyused for electrical discharge machining. Other sources are furnaces withcontrolled gaseous environments. The heat can be applied either:

(i) continuously, or

(ii) intermittently (pulsed)

by either controlling the source of heat or the exposure of the alloy tothe source of heat.

The processing can also first be accomplished in the metal alloy whichis then subsequently formed into the shape of the medical device orimplant.

The gaseous environment can be chosen to include elements that uponintroduction into the base alloy or device provide one or more of theembodiments described above.

Processing parameters such as time, temperature, gaseous environment canbe selected to create a microstructure in the alloy material thatproduces a MRI heating resistant surface structure comprising one ormore of the following layers generally in any order from the surface ofthe metal alloy or device:

(i) A layer such as that depicted in FIG. 3 according to an embodimentof the invention comprising a continuous or discontinuous lamellarnanostructure or microstructure as a result of a spinodal decomposition,eutectic or eutectoid reactions.

(ii) A layer such as that depicted in FIG. 4 according to an embodimentof the invention having a different level of crystallinity as comparedto a level of crystallinity the bulk of the composition (as defined bythe percentage of atoms that are arranged in a periodic and repeatingthree-dimensional array).

(iii) A layer such as that depicted in FIG. 5 according to an embodimentof the invention comprising a matrix phase having a plurality ofnanometer scale to micron scale particles, precipitates and/orinclusions therein, wherein the particles, precipitates or inclusionsdiffer in chemical composition and physical characteristics from thematrix phase and are discontinuously distributed therein.

(iv) A layer such as that depicted in FIG. 6 according to an embodimentof the invention containing one or more metal atoms different from thefirst and second metal atoms that have been diffused into the material.

One or more of the above (i)-(iv) can be provided generally in any orderfrom the surface of the alloy or device. As described above, each of thelayers can have a thickness of ≧3 nanometers typically up to severalcentimeters, and two or more layers can also be combined into one. Forexample, one or more metal atoms can be diffused into a matrix phasethat already has a plurality of nanometer scale to micron scaleparticles, precipitates and/or inclusions.

FIG. 3 shows a depiction of the elemental addition and process controlthat causes a spinodal decomposition to result in a continuous anddiscontinuous lamellar nanostructure or microstructure. See K. T. Moore,W. C. Johnson, J. M. Howe, H. I. Aaronson and D. R. Veblen, On theinteraction between Ag-depleted zones surrounding y plates and spinodaldecomposition in an Al-22 at. % Ag alloy, Acta Materialia 50 (2002)943-956.). The length scale of the features (layer thicknesses) can befrom nanometers to centimeters.

FIG. 4 shows a depiction of a NiTi alloy material that includes an outersurface layer having a concentration of inclusions in a volumefraction≧3% and a recast sub-surface layer below the surface layerhaving a different level of crystallinity as compared to the outersurface layer, according to another embodiment of the invention. Theouter surface layer may contain precipitates, inclusions and/orparticles in a volume fraction≧3% as a result of the controlled gaseousand/or temperature environment the processing was carried out in. Thelength scale of the features can be from nanometers to centimeters.

FIG. 5 is a depiction showing the microstructure of an outer surfacelayer according to an embodiment of the invention. The outer surfacelayer (thickness range from nanometers to centimeters) can be seen toinclude a plurality of precipitates, inclusions and particles. Thelength scale of the features can be from nanometers to centimeters.

FIG. 6 is a depiction showing the concentration profile of diffusant Coatoms into the matrix. See Hardness profile measurements in functionallygraded WC-Co composites by C. Larsson and M. Oden Materials Science andEngineering A 382 (2004) 141-149.

In some cases it may be desirable to remove a portion of the thicknessesof the outer surface layer and/or recast layer utilizing a removalprocess such as electropolishing. This purpose of surface removal is totailor the surface of the alloy to tune the penetration depth of the RFfield as well as the heating associated with currents induced by it soas to minimize heating while in an MRI environment.

By controlling the heating parameters and the gaseous and/or temperatureenvironment (heating and cooling rate, hold time, concentration of oneor more gases around the sample), one or more of the following for theMRI heating resistant surface structure can be controlled:

(i) the spacing and layer thickness of the continuous or discontinuouslamellar nanostructure or microstructure (e.g., see FIG. 3)

(ii) the level of crystallinity (e.g., see FIG. 4)

(ii) the distribution and size of precipitates, inclusions and particles(e.g., see FIG. 5);

(iii) the diffusion profile (e.g., FIG. 6)

so as to influence the penetration or skin depth of the incident RFfield during MRI and the subsequent propagation of the induced currents.It is generally desirable to have as much of the incident RF energyassociated with heating while in an MRI environment reflected away sothat there are almost no resulting induced currents in the alloy ordevice. An alloy composition described herein can control the skin depthand the propagation of the resulting current so as to achieve this. Asrecognized by the Inventors, the skin or penetration depth and the MRIvisibility can be influenced by the elements and/or structure presentthrough that depth. The propagation of the resulting current and theassociated heating is influenced by the impedance of the volume of thematerial through which the induced current propagates which is in turninfluenced by the resulting structure resulting from the heatingparameters and the gaseous and/or temperature environment (heating andcooling rate, hold time, concentration of one or more gases around thesample).

EXAMPLES

The following non-limiting prophetic example serves to illustrateselected embodiments of the invention. It will be appreciated thatvariations in proportions and alternatives in elements of the componentsshown will be apparent to those skilled in the art and are within thescope of embodiments of the present invention.

Consider a surgical needle. Standard metallurgical techniques known inthe metallurgical arts, but unknown in the medical imaging and surgicalarts, can be used to extrude and heat treat a bar of Al-22 wt. % Ag toproduce a lamellar structure, such as depicted in FIG. 3. The lamellarstructure in this example would extend through the bulk of the alloy.The alloy could then be extruded and electrical discharge machined (EDM)to fabricate a needle. EDM parameters (power and time) can be selectedto control the surface microstructure similar to that depicted in FIG. 4(which was for a NiTi alloy). The needle can then be tested using fourASTM standards to demonstrate MRI compatibility—F2052-06 for Measurementof Magnetically Induced Displacement Force on Medical Devices in the MREnvironment, F2213-06 for Measurement of Magnetically Induced Torque onMedical Devices in the MR Environment, F2182-02a for Measurement ofMeasurement of Radio Frequency Induced Heating Near Passive ImplantsDuring MRI and ASTM F2119-01 for Evaluation of MR Image Artifacts fromPassive Implants. Furnace heat treating in a gaseous environment can becarried out to diffuse atoms into the needle (such as in FIG. 6) toenhance MRI compatibility.

Consider the case of a current carrying conductor for implanting intothe human body (e.g., a pacemaker lead). The wires that constitute thelead can be heated in one or more gaseous environments in a series ofsteps using a laser beam or in a controlled atmosphere furnace so as toobtain a profile of diffusant atoms such as shown in FIG. 6 to provideimproved MRI compatibility, including a reduction of heating while in anMRI environment. In this application, the profile will generally belimited to a few microns in depth so as to not affect the currentcarrying capability of the inside of the wires. The wires can then betested using four ASTM standards to demonstrate MRIcompatibility—F2052-06 for Measurement of Magnetically InducedDisplacement Force on Medical Devices in the MR Environment, F2213-06for Measurement of Magnetically Induced Torque on Medical Devices in theMR Environment, F2182-02a for Measurement of Measurement of RadioFrequency Induced Heating Near Passive Implants During MRI and ASTMF2119-01 for Evaluation of MR Image Artifacts from Passive Implants. Thewires can then be wound into leads for implantation in the human body.

While various disclosed embodiments have been described above, it shouldbe understood that they have been presented by way of example only, andnot limitation. Numerous changes to the disclosed embodiments can bemade in accordance with the disclosure herein without departing from thespirit or scope of the invention. Thus, the breadth and scope of thisdisclosure should not be limited by any of the above describedembodiments. Rather, the scope of this disclosure should be defined inaccordance with the following claims and their equivalents.

Although embodiments of the invention has been illustrated and describedwith respect to one or more implementations, equivalent alterations andmodifications will occur to others skilled in the art upon the readingand understanding of this specification and the annexed drawings. Inaddition, while a particular feature may have been disclosed withrespect to only one of several implementations, such a feature may becombined with one or more other features of the other implementations asmay be desired and advantageous for any given or particular application.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a,” “an,” and “the” are intended to include the pluralforms as well, unless the context clearly indicates otherwise.Furthermore, to the extent that the terms “including,” “includes,”“having,” “has,”, “with,” or variants thereof are used in either thedetailed description and/or the claims, such terms are intended to beinclusive in a manner similar to the term “comprising.”

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

We claim:
 1. An MRI compatible medical device having at least onenon-magnetic metal alloy portion, wherein said metal alloy portioncomprises: a bulk portion including a first and at least a second metaldifferent from said first metal; a surface of said metal alloy portioncompositionally and/or structurally different from said bulk portionincluding an integral MRI heating resistant surface structure having athickness>3 nanometers, said MRI heating resistant surface structurecomprising at least one of: (i) a matrix phase comprising said first andsaid second metal having a plurality of nanometer scale to micron scaleparticles, precipitates and/or inclusions therein constituting a volumefraction>3%, wherein said particles, precipitates or inclusions differin chemical composition and physical characteristics from said matrixphase and are discontinuously distributed therein; (ii) a level ofcrystallinity that is at least 5% less as compared to a level ofcrystallinity in said bulk portion of said metal alloy portion; and(iii) one or more metal atoms different from said first and second metalhaving a concentration profile evidencing diffusion which varies as afunction of distance into said metal alloy portion.
 2. The medicaldevice of claim 1, wherein said metal alloy portion includes a lamellarnanostructure or microstructure.
 3. The medical device of claim 2,wherein said lamellar nanostructure or microstructure extends into bothsaid MRI heating resistant surface structure and said bulk portion ofsaid metal alloy portion.
 4. The medical device of claim 2, wherein saidone or more different metal atoms from said first and second metal is ina concentration of at least 0.1 atomic % for increasing MRI visibilityof said metal alloy portion.
 5. The medical device of claim 4, whereinsaid metal atoms different from said first and second metal comprise Au,Pt, Pd or Ag.
 6. The medical device of claim 1, further comprising abattery, wherein said metal alloy portion is in leads that are coupledto said battery for carrying current from provided said battery.
 7. Themedical device of claim 6, wherein said leads comprise cardiac leads fora cardiac device or neuro-stimulator leads for a neuro-stimulatordevice.
 8. The medical device of claim 1, wherein said MRI heatingresistant surface structure includes said (i) matrix phase comprisingsaid first and said second metal having a plurality of nanometer scaleto micron scale particles, precipitates and/or inclusions thereinconstituting a volume fraction>3%, wherein said particles, precipitatesor inclusions differ in chemical composition and physicalcharacteristics from said matrix phase and are discontinuouslydistributed therein.
 9. The medical device of claim 1, wherein said MRIheating resistant surface structure includes said (ii) level ofcrystallinity that is at least 5% less as compared to a level ofcrystallinity in said bulk portion of said metal alloy portion.
 10. Themedical device of claim 1, wherein said MRI heating resistant surfacestructure includes said (iii) one or more metal atoms different fromsaid first and second metal having a concentration profile evidencingdiffusion which varies as a function of distance into said metal alloyportion.